Data Mappers (like Doctrine2) help us a lot to persist data. Yet many projects are still struggling with tough questions: -Where to put business logic? -How to protect our code from abuse? Where to put queries, and how test them? Let’s look beyond the old Gang of Four design patterns, and take some clues from tactical Domain Driven Design. At the heart of our models, we can use Value Objects and Entities, with tightly defined consistency boundaries. Repositories abstract away the persistence. Encapsulated Operations helps us to protect invariants. And if we need to manage a lot of complexity, the Specification pattern helps us express business rules in the language of the business. These patterns help us evolve from structural data models, to rich behavioral models. They capture not just state and relationships, but true meaning. The presentation is a fast paced introduction to some patterns and ideas that will make your Domain Model expressive, unbreakable, and beautiful.

1. Unbreakable
Domain Models
@mathiasverraes

2. A Map of the World

3. London
3h train rides
Kortrijk, Belgium
Paris
Amsterdam

4. All models are wrong,
but some are useful.

5. I’m Mathias Verraes
I'm an independent
consultant.
I help teams build
enterprise web
applications.

6. Blog
verraes.net
!
Podcast with @everzet
elephantintheroom.io
!
DDD in PHP
bit.ly/dddinphp

7. Domain
Problem Space
Domain Model
Solution Space

8. Data Model ~= Structural Model ~= State
!
Domain Model ~= Behavioral Model
!

9. Protect your invariants

10. The domain expert says
“A customer must
always have an
email address.”
* Could be different for your domain
** All examples are simplified

11. Test fails
class CustomerTest extends PHPUnit_Framework_TestCase!
{!
/** @test */!
public function should_always_have_an_email()!
{!
!
$customer = new Customer();!
!
assertThat(!
$customer->getEmail(),!
equalTo('jim@example.com') !
);!
!
}!
}

12. Test passes
class CustomerTest extends PHPUnit_Framework_TestCase!
{!
/** @test */!
public function should_always_have_an_email()!
{!
!
$customer = new Customer();!
$customer->setEmail('jim@example.com');!
assertThat(!
$customer->getEmail(),!
equalTo('jim@example.com') !
);!
}!
}

13. Test fails
class CustomerTest extends PHPUnit_Framework_TestCase!
{!
/** @test */!
public function should_always_have_an_email()!
{!
!
$customer = new Customer();!
assertThat(!
$customer->getEmail(),!
equalTo(‘jim@example.com') !
);!
$customer->setEmail(‘jim@example.com’);!
!
}!
}

14. final class Customer!
{!
private $email;!
!
public function __construct($email)!
{!
$this->email = $email;!
}!
!
public function getEmail()!
{!
return $this->email;!
}!
}

15. Test passes
class CustomerTest extends PHPUnit_Framework_TestCase!
{!
/** @test */!
public function should_always_have_an_email()!
{!
!
$customer = new Customer(‘jim@example.com’);!
!
assertThat(!
$customer->getEmail(),!
equalTo(‘jim@example.com') !
);!
}!
}

16. Use objects as
consistency boundaries

17. final class ProspectiveCustomer !
{!
public function __construct()!
{!
// no email!
}!
}!
!
final class PayingCustomer !
{ !
public function __construct($email)!
{!
$this->email = $email;!
}!
}

18. Make the implicit
explicit

19. final class ProspectiveCustomer !
{!
/** @return PayingCustomer */!
public function convertToPayingCustomer($email)!
{ !
//...!
}!
}!
!
final class PayingCustomer !
{ !
//...!
}

20. The domain expert meant
“A customer must
always have a valid
email address.”

21. $customerValidator = new CustomerValidator;!
if($customerValidator->isValid($customer)){!
// ...!
}

22. Test fails
class CustomerTest extends PHPUnit_Framework_TestCase!
{!
/** @test */!
public function should_always_have_a_valid_email()!
{!
!
$this->setExpectedException(!
'InvalidArgumentException'!
);!
!
new Customer('malformed@email');!
!
}!
}

23. Test passes
final class Customer !
{!
public function __construct($email)!
{!
if( /* boring validation stuff */) {!
throw new InvalidArgumentException();!
}!
$this->email = $email;!
}!
}

24. Violates
Single Responsibility
Principle

25. Test passes
final class Email!
{!
private $email;!
!
public function __construct($email)!
{!
if( /* boring validation stuff */) {!
throw new InvalidArgumentException();!
}!
$this->email = $email;!
}!
!
public function __toString() !
{!
return $this->email;!
} !
}

26. Test passes
final class Customer!
{!
/** @var Email */!
private $email;!
!
public function __construct(Email $email)!
{!
$this->email = $email;!
}!
}

27. Test passes
class CustomerTest extends PHPUnit_Framework_TestCase!
{!
/** @test */!
public function should_always_have_a_valid_email()!
{!
!
$this->setExpectedException(!
‘InvalidArgumentException’!
);!
!
new Customer(new Email(‘malformed@email’));!
!
}!
}

28. Entity
!
Equality by
Identity
Lifecycle
Mutable
Value
Object
Equality by
Value
!
Immutable

29. Encapsulate
state and behavior
with Value Objects

30. The domain expert says
“A customer
orders products
and pays for them.”

31. $order = new Order;!
$order->setCustomer($customer);!
$order->setProducts($products);!
$order->setStatus(Order::UNPAID);!
!
!
// ...!
!
!
$order->setPaidAmount(500);!
$order->setPaidCurrency(‘EUR’);!
!
$order->setStatus(Order::PAID);!
!

32. $order = new Order;!
$order->setCustomer($customer);!
$order->setProducts($products);!
$order->setStatus(!
new PaymentStatus(PaymentStatus::UNPAID)!
);!
!
!
!
$order->setPaidAmount(500);!
$order->setPaidCurrency(‘EUR’);!
!
$order->setStatus(!
new PaymentStatus(PaymentStatus::PAID)!
);

33. $order = new Order;!
$order->setCustomer($customer);!
$order->setProducts($products);!
$order->setStatus(!
new PaymentStatus(PaymentStatus::UNPAID)!
);!
!
!
!
$order->setPaidMonetary(!
new Money(500, new Currency(‘EUR’))!
);!
$order->setStatus(!
new PaymentStatus(PaymentStatus::PAID)!
);

34. $order = new Order($customer, $products);!
// set PaymentStatus in Order::__construct()!
!
!
!
!
!
!
!
$order->setPaidMonetary(!
new Money(500, new Currency(‘EUR’))!
);!
$order->setStatus(!
new PaymentStatus(PaymentStatus::PAID)!
);

35. $order = new Order($customer, $products);!
!
!
!
!
!
!
!
!
$order->pay(!
new Money(500, new Currency(‘EUR’))!
);!
// set PaymentStatus in Order#pay()!
!

36. Encapsulate operations

37. $order = $customer->order($products);!
!
!
!
!
!
!
!
!
$customer->payFor(!
$order,!
new Money(500, new Currency(‘EUR’))!
);!
!

38. The domain expert says
“Premium customers
get special offers.”

39. if($customer->isPremium()) {!
// send special offer!
}

40. The domain expert says
“Order 3 times
to become a
premium customer.”

41. interface CustomerSpecification !
{!
/** @return bool */!
public function isSatisfiedBy(Customer $customer); !
}

42. class CustomerIsPremium implements CustomerSpecification !
{!
private $orderRepository;!
public function __construct(!
OrderRepository $orderRepository!
) {...}!
!
/** @return bool */!
public function isSatisfiedBy(Customer $customer) !
{!
$count = $this->orderRepository->countFor($customer);!
return $count >= 3;!
}!
}!
!
$customerIsPremium = new CustomerIsPremium($orderRepository)!
if($customerIsPremium->isSatisfiedBy($customer)) {!
// send special offer!
}!

43. $customerIsPremium = new CustomerIsPremium;!
!
$aCustomerWith2Orders = ...!
$aCustomerWith3Orders = ...!
!
assertFalse(!
$customerIsPremium->isSatisfiedBy($aCustomerWith2Orders)!
);!
!
assertTrue(!
$customerIsPremium->isSatisfiedBy($aCustomerWith3Orders)!
);!
!
!

44. The domain expert says
“Different rules apply
for different tenants.”

45. interface CustomerIsPremium !
extends CustomerSpecification!
!
final class CustomerWith3OrdersIsPremium !
implements CustomerIsPremium!
!
final class CustomerWith500EuroTotalIsPremium!
implements CustomerIsPremium!
!
final class CustomerWhoBoughtLuxuryProductsIsPremium!
implements CustomerIsPremium!
!
...!

46. final class SpecialOfferSender!
{!
private $customerIsPremium;!
!
!
public function __construct(!
CustomerIsPremium $customerIsPremium) {...}!
!
!
public function sendOffersTo(Customer $customer) !
{!
if($this->customerIsPremium->isSatisfiedBy(!
$customer!
)) !
{!
// send offers...!
}!
}!
}!

47. !
<!-- if you load services_amazon.xml: -->!
<service id="customer.is.premium"!
class="CustomerWith500EuroTotalIsPremium"> !
!
<!-- if you load services_ebay.xml: -->!
<service id="customer.is.premium"!
class="CustomerWith3OrdersIsPremium"> !
!
!
<!-- elsewhere -->!
<service !
id=”special.offer.sender”!
class=”SpecialOfferSender”>!
<argument type=”service” id=”customer.is.premium”/>!
</service>

48. Use specifications to
encapsulate rules
about object selection

49. The domain expert says
“Get a list of
all premium
customers.”

50. interface CustomerRepository!
{!
public function add(Customer $customer);!
!
public function remove(Customer $customer);!
!
/** @return Customer */!
public function find(CustomerId $customerId);!
!
/** @return Customer[] */!
public function findAll();!
!
/** @return Customer[] */!
public function findRegisteredIn(Year $year);!
}!

51. Use repositories to
create the illusion of
in-memory collections

52. interface CustomerRepository!
{!
!
/** @return Customer[] */!
public function findSatisfying(!
CustomerSpecification $customerSpecification!
);!
!
}!
!
!
// generalized:!
$objects = $repository->findSatisfying($specification);!

53. class DbCustomerRepository implements CustomerRepository!
{!
/** @return Customer[] */!
public function findSatisfying(!
CustomerSpecification $specification) !
{!
!
return array_filter(!
$this->findAll(),!
function(Customer $customer) use($specification) {!
return $specification->isSatisfiedBy($customer);!
}
!
);!
!
}!
}!

54. final class CustomerWith3OrdersIsPremium!
implements CustomerSpecification!
{!
public function asSql() {!
return ‘SELECT * FROM Customer...’;!
}!
}!
!
!
// class DbCustomerRepository !
public function findSatisfying($specification) !
{!
return $this->db->query($specification->asSql());
}
!

55. Use double dispatch
to preserve encapsulation

56. $expectedCustomers = array_filter(!
$repository->findAll(),!
// filter…!
);!
!
$actualCustomers = !
$repository->findSatisfying($specification);!
!
assertThat($expectedCustomers, equalTo($actualCustomers));

57. Test by comparing
different representations
of the same rule

58. Protect your invariants
!
Objects as
consistency boundaries
!
Encapsulate
state and behavior

59. Thanks! Questions?
I ♥ Feedback
joind.in/10690
!
Blog, Slides, other talks:
verraes.net
@mathiasverraes